The present invention relates to gas-assisted plastic injection molding and more particularly to an improved process and mold design for improved manufacture of hollow articles in a multiple cavity system.
There are numerous processes and systems known today for injection molding plastic articles. In a number of these systems and processes, a pressurized gas, such as nitrogen, is used in conjunction with the injection of the plastic material. The pressurized gas creates hollow interior portions in the molded article which result in savings in weight and material, thereby reducing costs. By reducing the thickness of the walls of the part, the use of pressurized gas also reduces the cycle time to manufacture the articles. The pressurized gas also applies an outward pressure to force the plastic material against the mold surfaces while the articles solidify, providing better surfaces on the molded articles and reducing or eliminating sink marks and other surface defects.
There are numerous problems and concerns with gas-assisted plastic injection molding, just as there are with conventional plastic injection molding. Injection molded articles typically shrink from their cavity size and the stresses in the material caused by the injection molding process often lead to distortion or warpage of the final products. Any nonuniform wall thicknesses in plastic parts made with gas-assisted plastic injection molding also can create stresses in the products which can lead to warping and twisting of the final products as they cool and solidify.
In both conventional plastic injection molding and gas-assisted plastic injection molding, it is common to use multiple cavities in a mold. This increases the number of plastic products that can be molded in each injection cycle, thus reducing cost and increasing productivity and revenue. Multiple cavity systems typically use manifolds, hot runner systems, and the like in order to deliver the plastic material to each of the mold cavities. This can create additional difficulties and concerns in the injection molding processes. Moreover, in multiple cavity gas-assisted plastic injection molding, it is often difficult to create products from each of the mold cavities which are exactly the same. Varying pressures, temperatures, and gas injection characteristics can produce products from a multiple cavity injection molding process which have differently sized and shaped hollow sections thereby creating some parts which have a greater tendency to warp or twist than others.
The above noted problems and molding difficulties are particularly noticeable when the articles being produced are elongated (that is, long and narrow) in shape. With elongated plastic molded articles, it is often difficult to have the gas migrate along the full length of the article or to have walls of uniform thickness. This often results in thicker plastic sections at locations along the length of the elongated article, particularly at one or both of the ends. The uneven thickness lengthens the cooling and product cycle, as well as creates products which have an increased tendency to warp or distort.
There are several quality and production issues that develop as a result of conventional approaches. There are aesthetic issues or surface imperfections that result from a lack of internal melt pressure during the fill of the mold cavity or by the non-uniform advance of the front of the melt in the cavity. Also, as indicated, there are dimensional issues that result from non-uniform residual wall thickness. This is due in part to flow imbalances in the mold cavity, as well as the volumetric requirements due to the flow imbalance. Also, in some conventional systems, there is an inability to vent the gas pressure from the molded article which, among other things, increases the cycle time.
It is an object of the present invention to provide an improved method of gas-assisted plastic injection molding. It is another object of the present invention to provide a mold design and process for multiple cavity injection molding of plastic parts using gas-assisted techniques.
It is still another object of the present invention to provide a mold design and process for gas-assisted injection molding which in a multiple cavity system results in production of more uniform products from all the cavities. It is a still further object of the present invention to provide a multiple cavity-assisted plastic injection molding system in which the plastic flows in a balanced manner in all of the mold cavities, and the resultant products have uniform wall thicknesses.
In accordance with the objects of the present invention, a multiple cavity gas-assisted plastic injection mold design and molding process are provided which overcomes problems with conventional multiple cavity mold designs and processes. The present invention has particular use with the molding of elongated hollow plastic products.
In accordance with the invention, the elongated mold cavities are all oriented in the vertical direction with the gate location being at the bottom of each of the mold cavities such that the direction of the melt front advance is vertical from the bottom of the cavity to the top. A valve gated hot runner system is utilized to isolate the gas pressure from the resin distribution system. The gas pin location is also adjacent the gate end of the mold cavity and also located adjacent to or forward of the resin entry point.
A preferred hot runner configuration is an xe2x80x9cXxe2x80x9d pattern which provides equal flow lengths of the plastic material to each of the valve gates. The sprue from the injection nozzle should be preferably directly in the center of the hot runner manifold. This results in the sprue bushing being offset from the centerline of the mold. This configuration provides optimum uniformity in dynamic flow of the resin to the multiple cavities.
In another embodiment of the invention, an xe2x80x9cH-patternxe2x80x9d hot runner member is utilized. This offsets the sprue and allows it to remain at the centerline of the mold. Although this embodiment can result in a flow imbalance, this can be compensated for artificially.
In the inventive process, a short shot of plastic material is injected into each of the cavities in the mold. The molten resin is injected by conventional injection methods into the cavity from the bottom of each of the vertically-oriented mold cavities. The amount of material injected into each of the mold cavities is balanced. When the plastic injection process is completed, the valve gates close in order to isolate each of the cavities. Pressurized gas is then introduced at the gate ends of each of the parts. This displaces the molten resin found in the interior of the parts and completely fills the mold cavities with the resin. Pressurized gas then preferably is provided to pack the molded articles in the mold cavities. Thereafter, once the parts are cooled and solidified, the gas is vented and the pressurized gas from the interior of the molded parts is removed. This reduces the gas pressure in the interiors of the molded parts. Finally, the mold is opened and the parts are ejected.
After the molded parts are removed from the mold cavities, the mold is closed, and the cycle is repeated.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiments for carrying out the invention when taken in accordance with the accompanying drawings.